Rotary electric machines, such as motor-generators, ISGs (Integrated Starter Generators), and so on, are operative to output, based on controlled AC (Alternating Current) power generated from DC (Direct Current) power supplied from a power source, torque, power, or the like. Power converters, such as inverters, are used to generate controlled AC power based on input DC power supplied thereto from the power source in order to control these controlled variables of the rotary electric machines.
PWM (Pulse Width Modulation) control or pulse control are known for a power converter, i.e. an inverter, to control AC power supplied to a rotary electric machine. For example, PWM control and pulse control are applied to an inverter for controlling AC power supplied to an ISG or a motor-generator in combination or alone.
PWM control applied to an inverter has higher controllability for input current. Particularly, PWM control applied to an inverter is capable of controlling the level of input current from a battery, i.e. a DC power source, when the rotational speed of the rotor of a controlled rotary electric machine is zero, i.e. the rotor of the controlled rotary electric machine is at a standstill or is stopped. However, PWM control applied to an inverter may necessitate a relatively high-capacitance capacitor connected to the input of the inverter to absorb ripples generated during PWM control, resulting in an increase of the inverter in size.
In contrast, pulse control, i.e. rectangular-wave control, applied to an inverter has no need of such a relatively high-capacitance capacitor connected to the input of the inverter because of few ripples generated during pulse control, resulting in the inverter having a smaller size.
However, pulse control applied to an inverter may have lower controllability for input current when driving a controlled rotary electric machine via the inverter with the rotational speed of the rotor of the controlled rotary electric machine being zero. This may result in a high level of current being pulled from a battery into the inverter, thus dropping the battery's voltage. The drop of the battery's voltage may have negative effects on other components operating based on the battery's voltage.
In addition, wires connected among the battery, the inverter, and the controlled rotary electric machine may have a relatively large thickness enough to permit such a high level current to flow therethrough, resulting in an increase of the wires in weight and in difficulty of wiring work using the wires. The inverter and the controlled rotary electric machine also have normal rated power enough to be acceptable thereby, resulting in an increase of the inverter and the controlled rotary electric machine in size.
Particularly, let us consider that pulse control is applied to an inverter installed in a motor vehicle that has a limited power-supply capacity. In this case, pulse control may have to ensure a minimum voltage of a power supply based on the battery if other components, such as an engine ECU (Electronic Control Unit), an EPS (Electronic Power Steering), brakes, and so on, are electrically connected to the same battery in addition to the inverter. This is because if the minimum voltage of the power supply were not ensured, this might deteriorate the fundamental operations of the vehicle, such as running, turning, and stopping. In order to reliably operate the other components, a step-up DC-DC converter and/or current-suppression relays may be provided in the motor vehicle. However, this may increase the total cost of the motor vehicle and/or necessitate, in the motor vehicle, an additional space for installation of the DC-DC converter and/or current-suppression relays.
On the other hand, there is known an example of power converting systems, i.e. rotary electric-machine systems, for improving the controllability of a rotary electric machine equipped with a single set of three-phase windings, which is disclosed in Japanese Patent Publication No. 5174617.
The power converting system disclosed in the Patent Publication sets a plurality of mode-state quantities of parameters for each of operation modes of the rotary electric machine; the plurality of mode-state quantities of the parameters are used to obtain information about a switching pattern for turning on or off switching elements of the inverter.
The power converting system performs a method of driving the inverter using the plurality of mode-state quantities as follows.
Specifically, the method obtains the mode-state quantities of the parameters for an actual operating mode of the rotary electric machine. Then, the method generates, based on PWM control or pulse control, turn-on and turn-off instructions for the respective switching elements of the inverter based on the obtained mode-state quantities of the parameters for the actual operating mode of the rotary electric machine.